Co-author: S. Shawn Wei (Washington University in St Louis; now at Scripps Institition of Oceanography)

Seismological studies
using land and ocean bottom seismographs can help constrain models of mantle
melting by imaging velocity anomalies resulting from the presence of partial
melt. Arc-backarc systems are particularly
interesting, as they involve both flux and decompression melting mechanisms,
variable water input into the melting process, and various levels of
interaction between arc and backarc. In
addition, an extensive dataset of petrological and geochemical measurements
also provide important constraints. Seismic tomographic results imaging arc-backarc
systems show extensive upper mantle regions with velocities that are too slow
to be explained without invoking partial melt. In the
Mariana arc, mantle seismic anomalies beneath the arc and backarc spreading
center are separated by a high velocity, low attenuation region at shallow
depths (< 80 km), implying distinct arc and backarc melting regions, with
the anomalies coalescing at greater depths. Large slow velocity anomalies delineate the
regions with significant melt, extending from 10-50 km depth beneath the
backarc and 40-80 km depth beneath the volcanic arc, consistent with final melt
equilibrium depths estimated from basalt thermobarometry. In the Lau basin, backarc spreading center basalts
show a rapid transition from MORB-like chemistry in the north to back-arc basin
basalts with strong water and slab-derived geochemical components in the south
as the distance between the spreading center and the volcanic arc is reduced. Slow seismic velocity anomalies beneath the
spreading center extend deeper and farther west in the north, suggesting that
partial melting occurs along an upwelling limb of mantle flow originating in
the ambient mantle west of the backarc, but this feature is missing in the
south, indicating that the southern ridge samples only mantle in the vicinity
of the subducting slab, consistent with its high water content. The
amplitude of the observed Lau backarc seismic anomalies have an inverse relationship
to inferred mantle water content, suggesting that water reduces the melt
porosity. Water may increase the
efficiency of melt transport and reduce porosity by lowering the melt
viscosity, increasing grain size through faster grain growth, or by causing a
different topology of melt within the mantle rock. A lower melt porosity for aqueous melts is
also consistent with the smaller amplitude seismic anomaly seen for the
water-rich volcanic arc melting regions compared to the backarc melt production
zone. Seismic attenuation studies show
very high shear attenuation beneath the backarc spreading center consistent
with high temperatures and partial melt.
Perhaps most surprisingly, we also observe strong bulk attenuation,
suggesting that partially molten mantle absorbs seismic energy with some poorly
understood dissipative process.

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